Treatment of WEHI 231 or CH33 immature B lymphocytes with antisera against their expressed surface IgM (anti- IgM), which serves as B Cell Receptor (BCR), induces G1/S phase growth arrest followed by apoptosis. These cell lines have served as models for the study of tolerance via clonal deletion, and have provided important insights into mechanisms leading to death of B cell neoplasias. Work by several laboratories has demonstrated the drop in c-Myc oncogene expression resulting from BCR engagement promotes cell death. In this application, studies are focused on elucidation of how members of the c-Myc/Max/Mad network exert their affects. Work by the PI's laboratory has implicated both initiator (Inr) and E-box regulatory pathways in apoptosis mediated by anti-IgM treatment. Specifically, it has been shown: 1) the induction in p27Kip1 levels is due to both transcriptional and posttranscriptional mechanisms; 2) c-Myc can repress transcription of the p27 gene; 3) c-Myc can bind to the Inr element of the p27 gene in the presence of Max; 4) Forkhead FKHRL1 factor is induced following anti-IgM treatment; 5) transactivation of p27promoter by FKHRL1 can be repressed by c-Myc; 6) the drop in c-Myc is followed by an increase in expression of Max negative regulatory binding partner Mad1 and a decline in E-box gene expression. 7) Inhibition of ornithine decarboxylase (ODC) promotes apoptosis. These findings lead to the hypothesis that the drop in c-Myc following BCR engagement promotes cell death via two pathways: release of repression of Inr-mediated transcription and induction of repression of E-box-mediated gone expression. Experiments are proposed to: Aim 1) Elucidate the mechanisms leading to increased p27expression. Studies will be performed on the transcriptional activation by Forkhead proteins, the mechanism of repression by c-Myc, the involvement of Max binding partners, post-transcriptional control of p27, and identification of other Inr-containing genes de-repressed upon the drop in c- Myc. Findings on the mechanism of regulation will be extended to Jurkat T, Hs578T breast cancer, and PC12 pheochromocytoma tumor cell lines. Aim 2) Elucidate the role of induction of Max negative regulatory partners in repression of E-box gene transcription. Studies will be performed to characterize the changes in Max binding partner expression, and their involvement in repression of E-box regulated gene expression and the role of ODC in cell death. These findings will enhance our understanding of the mechanism of action of the c-Myc/Max/Mad network in signaling pathways that control expression of genes promoting apoptosis of tumor cells.